docs: def _sample_orthonormal_to(mu):
      """Sample point on sphere orthogonal to mu."""
      v = np.random.randn(mu.shape[0])
      proj_mu_v = mu * np.dot(mu, v) / np.linalg.norm(mu)
      orthto = v - proj_mu_v
      return orthto / np.linalg.norm(orthto)